The difference between sakura, fire, red cherry, etc. is basically the look. More colorful shrimp will cost more, otherwise they are exactly the same in terms of care, behavior, etc.
Culls are excess shrimp that someone is removing from their aquarium. Sometimes people cull at random and you could get some very nice colors, especially if the breeding stock was of high quality. Sometimes people cull selectively, removing the least colorful from their tank (though these could still be of pretty good quality if they started with all highly colored or very well patterned shrimp).
There are other colors as well. Cherry shrimp come in blue, green, black, chocolate, yellow, orange and bi-colored.
Just be aware red crystal shrimp and black crystal shrimp are a different species from cherry shrimp. They're really similar for the most part, just not the same.
You can get a group of 10 to 12 for around $30-$35 shipped to you from an online seller if you aren't trying to get extremely rare colors or patterns. I was actually browsing E Bay for them last night, and found a ton of sellers there too. Local fish shops and large retailers like petsmart also carry them, usually for between $2.50 and $5 each, depending on the store and the variety of shrimp. You can start with just 4 or 5 that way if you don't want to drop $30 all at once, but online is a bit less expensive per shrimp for nicer varieties.
